Police said the woman was out with friends in the State Street area and was grabbed from behind while walking on Fahrenbrook Court.

The woman said the man threatened to shoot her but no weapon was seen.

"She's very fearful that she's going to be killed," Madison Police Department spokesman Joel DeSpain said. "In the end, she was not physically injured in the sense that she wasn't beaten, shot, stabbed, anything like that, but one can only guess the traumatic stress she's under now having undergone such a stressful event."

The man fled south on North Mills Street after the assault.

"This is a serious stranger sexual assault, and so we want people to really be aware walking in the downtown all the time, but particularly right now because we have a very dangerous criminal out there," DeSpain said.

The man is described as black, 6 feet tall with a muscular build, short hair and facial stubble. He was wearing a light-colored T-shirt and shorts.

Anyone with information is asked to call Madison Area Crime Stoppers at 608-266-6014.